1. Field of the Invention:
This invention relates to a dc discharge lamp suitable for use as a light source, for example, in the ultraviolet ray exposure step of a fabrication process for semiconductors.
2. Description of the Prior Art:
In the fabrication of semiconductors for example, ultraviolet rays are used to print circuit patterns on silicon wafers. As light sources for giving off ultraviolet rays, super high pressure mercury vapor lamps are usually used.
In one example of super high pressure mercury vapor lamps which have conventionally been used in the fabrication of semiconductors, the super high pressure mercury vapor lamp is, as illustrated by way of example in FIG. 4, composed of a quartz glass envelope 1, an anode rod 4 and a cathode rod 5 extending at one ends thereof into the interior of the envelope 1 and fixedly disposed at the other ends thereof in an anode-sealing portion 2 and a cathode-sealing portion 3 respectively, a sealed anodic conductor 6 and a sealed cathodic conductor 7 embedded hermetically in the anode sealing portion 2 and cathode-sealing portion 3 respectively and formed of molybdenum foils which are respectively connected at one ends thereof to the anode rod 4 and cathode rod 5, an outer anodic lead 8 and an outer cathodic lead 9 provided fixedly in the anode-sealing portion 2 and cathode-sealing portion 3 respectively in such a way that the leads 8,9 are connected at one ends thereof to the sealed conductors 6,7 respectively and the other ends of the leads 8,9 project out from the anode-sealing portion 2 and cathode-sealing portion 3 respectively, and a metallic anode base 10 and a metallic cathode base 11 connected electrically to the leads 8,9 respectively and provided fixedly on the anode-sealing portion 2 and cathode-sealing portion 3 respectively. Within the envelope 1 of the super high pressure mercury vapor lamp, mercury and an inert gas are contained. Upon operation, the internal pressure of the envelope 1 increases to a pressure, for example, as high as about 10-50 atms.
Molybdenum foils are used as the sealed conductors 6,7, because the molybdenum foils can undergo plastic deformations to facilitate the maintenance of air tightness at the sealing portions 2,3 even for significant temperature variations to be induced by repeated turn-on and turn-off of the lamp. From the viewpoint of maintenance of air tightness, the thinner the metallic foils 6,7, the better. However, the thickness may generally be several tens micrometers or preferably, about 20 .mu.m or so when the electrical capacity, mechanical strength or the workability upon manufacture of such mercury vapor lamps is taken into consideration.
However, such conventional supervoltage mercury lamps have been found to involve a problem that even where their service life periods had been designed to reach, for example, 400 hours, they failed in many instances in about 100 to 300 hours; and due to such prematured failure, the ultraviolet exposure processing was frequently interrupted during the ultraviolet ray exposure step of a fabrication process of semiconductors and the smooth accomplishment of the fabrication process of the semiconductors was hence impaired.
With the foregoing in view, the present inventor made an investigation on super high pressure mercury vapor lamps which had failed prematurely. The investigation resulted in a finding that as shown partly on an enlarged scale in FIG. 5, a gap 32 was formed between the sealed conductor 7 and a glass portion 31 of the cathode-sealing portion 3 and only in the cathode-sealing portion 3. Namely, even if the gap 32 is small in the beginning, it appears to propagate little by little toward the outer cathodic lead 9 as the operation time becomes longer. Accordingly, the air-tight structure between the sealed conductor 7 and the glass portion 31 of the cathode-sealing portion 3 is eventually broken and the super high pressure mercury vapor lamp hence becomes inoperative. The present invention has been completed on the basis of such a finding.